The GSM-EDGE and UMTS standards place an increasingly stringent requirement on the linearity of handset transceiver circuits, particularly given the proposed wide channel band widths of handset transceivers compared to, for example, DAMPS and PDC systems. In order to realise a power efficient handset design, some form of linearisation will be required in the handset transceiver. This should be low power itself, capable of broadband linearisation (up to 5 MHz for UMTS/UTRA), frequency flexible, preferably multiband, and capable of achieving and maintaining high levels of linearity improvement when used to reduce distortion caused by highly non-linear power amplifiers (e.g. class-C amplifiers).
The use of conventional single channel linearisation schemes (e.g. Cartesian loop) suffer in that they are capable of controlling linearisation only over a finite gain-bandwidth. Such systems are not capable of providing the required linearity improvements (“gain”) over the channel bandwidths envisaged (for UMTS, in particular).
Alternative broadband linearisation schemes, such as feed-forward arrangements, are difficult to implement as integrated circuits. They are also unable to attain desired levels of power efficiency because they employ additional linear amplifiers to perform distortion cancellation.